Generally, a power beating device uses a driving unit, such as a liner movement of an electromagnet or a simple harmonic motion from the rotation movement of a motor, to resist against an energy storage unit, for example to use a spring to cause an impact unit to move from a first position to a second position so as to store the potential energy from electric energy. When the spring moves to a second position, the storage energy is converted into the dynamic energy of the impact unit so as to drive a nail into a work piece. The requirement of this kind of power beating device is that it must be light and power saved. However the defects of the electromagnetic driving are heavy and power consumed, while the simple harmonic motion from the rotation movement of a motor is that the power is unstable, especially when the impact unit moves to an impact position driving the nail, the spring needs a great driving force, but at this moment, the simple harmonic motion from the rotation movement of a motor is at a maximum value in the sinusoidal moving curve. To compensate this defect, in U.S. Pat. No. 4,811,885 A1, in the simple harmonic motion from the rotation movement of a motor, a flywheel is added for providing a greater pushing force to the spring as the spring moves to a distal end for driving the impact unit to beat the nail. However the flywheel consumes more power and is uneconomic.
In U.S. Pat. No. 4,953,774, another power beating device is disclosed, in that in the simple harmonic motion from the rotation movement of a motor, the spring has different pitches so as to improve the phase deficiency in the simple harmonic motion from the rotation movement of a motor. Furthermore, a cam unit is provided. The cam unit can reset the impact unit as in the distal end, the driving force is insufficient so that the impact unit is buckled in a dead point, while the power nail driver is power off. However all these devices will make the device complicated and can not improve the prior art defect effective.